This invention relates to an improved method and apparatus for making ophthalmic lenses. In particular, this invention is suited to molded ophthalmic lenses such as hydrogel contact lenses, although the method is also suitable for other small, high-precision ophthalmic lenses such as intraocular lenses and other methods of manufacturing such as by spin casting.
Soft ophthalmic lenses for placement on the cornea or within the eye, such as contact lenses or soft intraocular lenses, can be made by a variety of techniques. Ophthalmic lenses can be made by spin casting a monomer material in a rotating mold then polymerizing the material so shaped. Another method used to manufacture both contact lenses and intraocular lenses is precision lathing of a piece of material which is then polished and used as a lens.
Recently the molding of soft contact lenses and soft intraocular lenses has come into favor. This technique has the advantages of repeatability and speed that compares favorably with the prior methods of manufacturing lenses. Techniques for successfully molding hydrogel lenses can be found in U.S. Pat. Nos. 4,495,313 and 4,640,489 to Larsen and 4,889,664; 4,680,336 and 5,039,459 to Larsen et.al. These patents specifically described the use of acceptable monomers, a diluent which substitutes for water during the molding process and is replaced with water after the molding has been completed. The advantage of this technique is that the optical properties, size and shape of the lens thus made does not change as radically as with methods that do not utilize such diluent.
It is further known in the art to mold such ophthalmic lenses by forming a monomer or monomer mixture in a mold such as one made from polystyrene or polypropylene. An example of this art can be found in U.S. Pat. No. 4,565,348 to Larsen wherein the requirement for a polystyrene mold materials, chemistry and processes are discussed. In contrast to the above polystyrene molds, another example is the use of polypropylene or polyethylene molds such as that described in U.S. Pat. No. 4,121,896 to Shepherd.
A particular problem, however, is that the monomer or monomer mixture usually contains dissolved gases from the air (O.sub.2 and N.sub.2) that may cause at a minimum bubbles from inert gas, or interfere with polymerization if the gas is reactive with free radicals available during polymerization.
It has been recognized that in the manufacture of ophthalmic lens, particularly contact lenses, it is desirable to eliminate oxygen from the monomer mix, because oxygen interferes with the polymerization reaction. This is found to still be true with the production of molded contact lenses using a diluent. One practice is to degas the monomer or monomer mixture placing the monomer mixture into a rotary evaporator unit (such as the Rotovap available from Buchi Rotavapor, Inc. of Flawil, Switzerland; sold by Fisher Scientific of Springfield, N.J.) to remove excess gas. This procedure for instance, is applicable to the monomer mixtures described in U.S. Pat. Nos. 4,889,664 and 4,495,313 where the mixture is rotated under subatmospheric pressure. The container with a monomer mixture is then flushed with nitrogen and held under a nitrogen atmosphere until it is used.
This is done in a round flask half filled with monomer. The Rotovap unit spins the monomer to increase the surface and the reduction in gas is proportional to the ratio of the lower pressure to atmospheric pressure, that is, 760 mm Hg.
The overall gas content is reduced to the above ratio, but the N.sub.2 to O.sub.2 ration remains the same as in air.
Because the container is then back filled with N.sub.2 nitrogen gas has an opportunity to redissolve in the monomer mixture. The end result of the process is then actually an O.sub.2 removal process. While this procedure solves the problem of O.sub.2 reaction with the monomer and polymerization process, it does not eliminate problems associated with dissolved nitrogen which may cause bubbles to form during polymerization. In addition, once the monomer is reduced in oxygen content, exposure to the oxygen in the air during subsequent handling can cause O.sub.2 reabsorption.
In addition, at the low pressures (less than 40 mm Hg) volatile components of the monomer mixture may evaporate changing the composition of the monomer.
Finally, there are additional handling and manufacturing problems associated with maintaining a nitrogen environment around the gas-reduced monomer mixture produced in a batch process.
It is, therefore, an object of the present invention to greatly reduce the amount of dissolved oxygen in the monomer mixture used to produce ophthalmic lenses.
It is a further object of the invention to reduce the amount of dissolved nitrogen in the monomer mixture used for ophthalmic lens production.
It is a further object of the invention to reduce or eliminate the need for handling N.sub.2 gas during monomer processing and handling.
Another object of the invention is to minimize the evaporation of volatile components from the monomer mixture during dissolved gas removal.
Finally, it is an object of the invention to minimize the exposure of the degassed monomer mixture to atmospheric conditions, particularly oxygen, before being used to produce an ophthalmic lens. Additionally, it is desired to eliminate the need to perform a degassing operation on a batch basis in order that the degassed monomer is used as it is available further reducing handling and oxygen exposure time.